1. Field of the Invention
The present invention relates to an integrated external-modulator semiconductor laser device capable of operating at high speed for use in a transmitter for a wavelength multiplexed transmission system, and relates more specifically to such a semiconductor laser device selecting a plurality of wavelengths, and to a drive method for the semiconductor laser device.
2. Description of the Related Art
Wavelength-division multiplexing (WDM) techniques have been recently developed as a means of increasing the transmission capacity of fiber optic transmission paths. A particular benefit of WDM techniques is that the transmission capacity of existing fiber optic cables can be readily increased by several factors of ten. Semiconductor laser devices using an integrated modulator semiconductor laser, i.e., a semiconductor laser integrated with a modulator, have been developed as a light source for systems using WDM techniques. This type of semiconductor laser device dc drives a distributed feedback (DFB) laser diode and uses an electroabsorption modulator for high speed modulation of light emitted from the semiconductor laser.
The operating principle of an electroabsorption modulator is described briefly below.
The absorption layer of the electroabsorption modulator has either a multiquantum well or a strained multiquantum well structure. When a reverse field is applied to the absorption layer, the energy level of excitons (an electron and hole pair in a bound state) trapped inside the quantum well changes. This phenomenon is known as the quantum trapping Stark effect, and is illustrated in FIG. 31. The exciton causes a change in the absorption wavelength, which produces a change in light transmission. FIG. 32 illustrates exciton-induced change in absorption wavelength. As shown in FIG. 32, when there is a change in exciton energy level, the absorption wavelength of the exciton shifts to a longer wavelength. This effect is used for light modulation.
Referring to FIG. 32, if the oscillating wavelength of the laser is xcex, absorption coefficient a of wavelength xcex changes xcex94a depending on whether or not a reverse field is applied to the modulator. When a field is not applied, light passes the modulator, but when a reverse field is applied the modulator absorbs light from the laser. Considering exciton energy shift, it is also necessary to set the equivalent band gap wavelength of the modulator absorption layer to a shorter wavelength. A typical laser oscillation wavelength is approximately 1550 nm, and the corresponding band gap wavelength of the modulator absorption layer is approximately 1500 nm. The difference between these wavelengths, xcex94xcex, is thus approximately 50 nm, and the equivalent band gap wavelength of the modulator absorption layer must therefore be set to a wavelength that is xcex94xcex shorter than the laser oscillation wavelength.
FIG. 33 is a typical view of an exemplary integrated modulator semiconductor laser device according to the related art. FIG. 34A shows the offset bias input to an optical modulator 102, and FIG. 34B shows the light output from the optical modulator 102. Referring to FIGS. 33, 34A, and 34B, it is to be noted that the optical modulator 102 and semiconductor laser 103 are monolithically integrated on semiconductor substrate 101, forming semiconductor laser chip 104. This chip 104 is affixed to metallic block 106 by way of intervening submount 105 of SiC or other material, and to metal package 108 by way of an intervening temperature control Peltier element 107 affixed to the opposite side of metallic block 106.
The Peltier element 107 is a solid cooling element that uses the Peltier effect commonly used for temperature control. The Peltier element 107 can be controlled to either heat or cool by changing the polarity of current flowing to the element 107. A thermistor (not shown in the figure) is used to detect the temperature of the semiconductor laser chip 104. Based on the temperature detected by this thermistor, the current flowing to the Peltier element 107 is controlled to maintain the semiconductor laser chip 104 at a desired temperature. The semiconductor laser 103 is driven to maintain a constant output by applying voltage to the semiconductor laser 103 in the forward direction.
The optical modulator 102 is driven by a high frequency square wave signal voltage. Light input to the optical modulator 102 from the semiconductor laser 103 is then modulated by the optical modulator 102 to a light wave output corresponding to the signal voltage. For example, when a normally 2- to 3-V reverse voltage is applied to the optical modulator 102, light is absorbed and laser beam emissions from the modulator 102 stop. A desirable offset voltage Voffset of typically less than 1 V, for example, may be applied in some cases to the optical modulator 102 when in a light transmitting state.
As the technology has progressed and the degree of wavelength multiplexing has increased, a new type of light source that does not use a single integrated modulator semiconductor laser to provide all wavelengths used by the WDM system has been needed. Examples of such light sources include a variable wavelength light source that uses a single semiconductor laser chip to output at multiple wavelengths, and an array of multiple wavelength semiconductor lasers integrated on a single chip. In addition to achieving a light source that could be used in a variety of systems, significant cost benefits could be obtained by providing many light sources in a single chip for use as a backup light source for a WDM transmission device. Such multiple wavelength light sources can also be used to route signals through a network using a wavelength routing technique by tuning laser output to a specific wavelength at different points in the network, and will be needed to build all types of optical networks in the future.
As a result, there has been much research into variable wavelength light sources using a single chip to output multiple wavelengths of light, as well as devices that integrate an array containing a plurality of semiconductor lasers of different wavelengths into a single light source chip, and integrate a modulator for each of the multiple light sources on this chip. For example, xe2x80x9cSix-Channel WDM Transmitter Module with Ultra-Low Chirp and Stable xcex Selection,xe2x80x9d ECOC 1995 Th.B.3.4, from Lucent Technologies describes a device shown in FIG. 35. This device integrates an array of six semiconductor lasers, each producing a different wavelength at a 1.6 nm wavelength interval, with a coupler, semiconductor amplifier, and modulator. The semiconductor laser chip shown in FIG. 35 operates by selecting any one of the six output wavelengths.
As described above, the difference xcex94xcex between the oscillation wavelength of the semiconductor laser and the band gap wavelength of the optical modulator has a significant effect on the absorption characteristic of the optical modulator, and by extension has a significant effect on transmission characteristics at the oscillation wavelength of the semiconductor laser. The performance of the above-noted Lucent device when transmitting at 2.5 GHz over a 600 km path (Voffset=xe2x88x921.5 V, 3.5 Vp-p) was evaluated using two wavelengths offset 3.2 nm. Minimum reception sensitivity at a bit error rate (BER) of 1e-9 was xe2x88x9231.2 dBm at 1559.71 nm and xe2x88x9230.4 dBm at 1556.49 nm, and its wavelength dependency is expected to be high.
The paper xe2x80x9cWidely Tunable Sample Grating DBR Laser with Integrated Electroabsorption Modulator,xe2x80x9d published by UCSB (University of California, Santa Barbara) (PTL Vol. 11 No. 6, 1999, pp. 638-640) describes a device having a tunable sampled grating DBR laser capable of producing 51 different wavelengths at 0.8 nm intervals integrated with a modulator. As in the above device, the modulator uses a bulk active layer (band gap wavelength xcex94g=1.43 xcexcm), and was evaluated for the wavelength dependence of the extinction ratio only. Wavelength dependence was shown to be quite high.
In order to modulate a wide range of wavelengths, a bulk absorption layer must be used in the modulator when a single electroabsorption modulator according to the related art is used to couple and modulate laser beams of multiple wavelengths. However, difference xcex94xcex varies with wavelength. even when such a bulk absorption layer is used, and it is difficult to achieve a device capable of operating across a wide wavelength range, particularly a wavelength range exceeding 12 nm, because of different transmission characteristics.
With consideration for the above mentioned problem, it is therefore an object of the present invention to provide a semiconductor laser device using an integrated modulator, multiwavelength semiconductor laser capable of modulation with the same wavelength difference xcex94xcex across a wide wavelength range, such as 12 nm or greater, using a single optical modulator.
It is a further object of the present invention to provide a drive method for this semiconductor laser device.
To achieve the object of the present invention, a semiconductor laser device according to a first preferred embodiment includes a laser unit having a plurality of single oscillation wavelength semiconductor lasers each having a different oscillation wavelength. An optical modulation unit has one electroabsorption modulator for optically modulating and externally emitting laser light from the laser unit. A coupling unit couples the laser unit and optical modulation unit so that laser light from each semiconductor laser is incident to the electroabsorption modulator. A temperature control unit controls the temperature of the laser unit and optical modulation unit by heating or cooling the respective unit. A laser selection unit selects and operates one of the plurality of semiconductor lasers in the laser unit. An offset bias control unit controls applying an offset bias to the electroabsorption modulator of the optical modulation unit. A controller specifies for the laser selection unit the semiconductor laser to be operated based on an external command, and controls operation of the temperature control unit and the offset bias control unit to attain a constant specified wavelength difference between the oscillation wavelength of the specified semiconductor laser and the band gap wavelength of the electroabsorption modulator.
This semiconductor laser device applies as needed an offset bias to the electroabsorption modulator, and controls the temperature of the laser unit and optical modulation unit so that the wavelength difference is the same between the band gap wavelength of the electroabsorption modulator and the wavelength emitted from the selected semiconductor laser. This makes it possible for a single modulator operating in a single state to modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater.
Another version of the invention provides a semiconductor laser device including a laser unit having a single-wavelength semiconductor laser capable of oscillating at a plurality of different wavelengths. An optical modulation unit has one electroabsorption modulator for optically modulating and externally emitting laser light from the laser unit. A coupling unit couples the laser unit and optical modulation unit so that each laser output from the semiconductor laser is incident to the electroabsorption modulator. A temperature control unit controls the temperature of the laser unit and optical modulation unit by means of heating or cooling the respective unit. A wavelength selection and drive unit drives the semiconductor laser of the laser unit at a specific desired oscillation wavelength. An offset bias control unit controls applying an offset bias to the electroabsorption modulator of the optical modulation unit. A controller specifies for the wavelength selection and drive unit what oscillation wavelength the semiconductor laser is to be driven at based on an external command, and controls operation of the temperature control unit and the offset bias control unit to attain a constant specified wavelength difference between the specified oscillation wavelength and the band gap wavelength of the electroabsorption modulator.
This semiconductor laser device appropriately applies an offset bias to the electroabsorption modulator, and controls the temperature of the laser unit and optical modulation unit so that the wavelength difference is the same between the band gap wavelength of the electroabsorption modulator and the wavelength emitted from the variable oscillation wavelength semiconductor laser. This makes it possible for a single modulator operating in a single state to modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater, even when a single variable wavelength semiconductor laser is used.
Whichever configuration is used, the laser unit, coupling unit and optical modulation unit are preferably integrated on a semiconductor substrate to form a semiconductor laser chip. In this case, the temperature control unit controls the temperature of the semiconductor laser chip. This makes it possible for a single modulator operating in a single state to modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater, even when using a semiconductor laser chip.
It is alternatively possible for the laser unit, coupling unit and optical modulation unit to be packaged on the same substrate and form a hybrid integrated circuit. In this case, the temperature control unit controls the temperature of the hybrid integrated circuit. This makes it possible for a single modulator operating in a single state to modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater, even when using a hybrid integrated circuit.
In a further alternative version of the present invention the laser unit, coupling unit and optical modulation unit are preferably discrete modules. In this case the temperature of each module is controlled by the temperature control unit. It is also possible with this configuration for a single modulator operating in a single state to modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater. In addition, this semiconductor laser device can be used in an optical communications system using an optical transmitter.
Further preferably, the electroabsorption modulator of the optical modulation unit has a light absorption layer with a multiquantum well structure. When thus comprised, a single modulator operating in a single state can modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater, without using a bulk active layer. The same benefit can also be achieved using a strained multiquantum well structure instead of this multiquantum well structure.
The invention is further directed to a semiconductor laser device drive method.
This drive method is used with the first semiconductor laser device described above according to the invention. This control method controls the temperature of the laser unit and optical modulation unit, and controls applying an offset bias to the electroabsorption modulator of the optical modulation unit, to attain a constant specified wavelength difference between the oscillation wavelength of a semiconductor laser in the laser unit and the band gap wavelength of the electroabsorption modulator.
By thus controlling the offset bias applied to the electroabsorption modulator and the temperatures of the laser unit and optical modulation unit so that the wavelength difference is the same between the band gap wavelength of the electroabsorption modulator and the oscillation wavelength of each semiconductor laser, a single modulator operating in a single state can modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater.
A semiconductor laser device drive method according to another version of the invention is used with the second semiconductor laser device described above according to the present invention. This control method controls the temperature of the laser unit and the optical modulation unit, and controls applying an offset bias to the electroabsorption modulator of the optical modulation unit, to attain a constant specified wavelength difference between the oscillation wavelength of the semiconductor laser in the laser unit and the band gap wavelength of the electroabsorption modulator.
By thus controlling the temperature of the variable single-wavelength semiconductor laser and the electroabsorption modulator, and controlling the offset bias applied to the electroabsorption modulator so that a constant specified wavelength difference is maintained between the oscillation wavelength of the variable single-wavelength semiconductor laser and the band gap wavelength of the electroabsorption modulator, a single modulator operating in a single state can modulate multiple laser output wavelengths selected from a wide wavelength range, such as 12 nm or greater, even using a variable single-wavelength semiconductor laser.